The invention relates to a seal between an inner member and an enveloping outer member, such as a shaft and an enveloping housing, which are rotatable with respect to each other. The seal contains a viscous medium such as oil or grease which separates a space containing a gas of higher pressure from a space containing a gas of lower pressure. A seal of this kind can be used, for example, in hot-gas reciprocating engines in which the crank shaft is accommodated in a crank case containing gas under pressure (for example 100 atmospheres), the crank shaft being passed through a crank case wall to the surroundings where atmospheric pressure prevails.